The present invention relates to fiber optic pressure sensors and is particularly directed to an apparatus that permits pressure sensing by sensing the change in evanescent coupling between two optical fibers.
The prior art teaches various methods for accomplishing pressure sensing using optical fibers. One such method transmits a source signal through an optical fiber and directs the source beam against a reflective membrane. The reflective membrane is in communication with the acoustic or pressure signals to be monitored. Another optical fiber is located in such a position as to receive the reflected signal from the reflective membrane. When the reflective membrane is subjected to pressure signals the membrane will vibrate, thus causing the angle of reflection at the membrane to vary and thus the reflected light will change. The reflected light is partially collected by the second optical fiber which then transmits the changing optical signal back to a decoding system.
Another system taught by the prior art for detecting pressure waves using optical fibers is to vary the optical coupling coefficient between two fibers in response to the pressure waves. An input fiber carries a steady state optical signal which may either be continuous or pulsed. This input signal is then coupled to an output optical fiber via a coupling network. The coupling means is subject to the pressure variations such that when pressure occurs on the coupling means the energy will no longer be in direct coupling with the output fiber. Such a system is taught in U.S. Pat. No. 4,071,753 to Fulenwider et al.